Abstract: A new method of fracture observation was used cout nearly 20 different degree of metamorphism coal polished sections. The relationship between submicroscopic fracture density and coal rank was found through the observation. when R_O,max<1.3, the density of fracture increases with the coal rank; the maximum of fracture density appeares when R_O,max=1.3 and reaches 60 fractures per 10 cm; when the R_O,max>1.3, fracture density decreases with the coal rank. The mechanism for above phenomenon is explained at the same time.